Democratic elites were clear more than a century ago about the why they wanted runoff elections: to tighten their grip on power and keep black voters out of the political system.

Today, the system has flipped. Now it’s Mississippi Republicans who face a runoff June 24, and a scheme crafted to protect the powerful might very well defeat them, just as it has throughout the South in recent years.

Primary election runoffs _ still held in seven states, almost all in the South _ were devised around the turn of the 20th century as a way of making sure that “unsavory” candidates couldn’t break through and win. That meant marginalizing dissident voters, notably blacks.

“When the majority of Southern whites were Democrats, the runoff primary promoted white Democratic supremacy, and it was adopted with the intention to do so,” said Morgan Kousser, a California Institute of Technology professor of history and social sciences who’s written extensively on runoffs.

South Carolina was the first to embrace the runoff, and the idea quickly spread. In 1904, Mississippi state Sen. Edmond Noel, who’d later became governor, explained his view of the state’s primary system.

“White supremacy could be maintained only by the members of that race remaining together politically,” he wrote. “Otherwise comparatively few Negroes who are qualified to vote might wield the balance of power.”

Modern runoffs have taken a different twist: They’ve become battles of insurgents, usually with conservative ideological agendas, fighting an entrenched Republican status quo.

In Mississippi, state Sen. Chris McDaniel is the latest example. The tea party favorite faces Sen. Thad Cochran, the six-term organization man, on June 24 for the Republican Senate nomination.

The next runoff comes July 22 in Georgia’s Republican Senate contest, where U.S. Rep. Jack Kingston, the establishment favorite, vies with businessmen David Perdue after neither could muster a majority in their first round.

Modern history suggests the odds favor the challengers.

Recent challengers who didn’t reach 50 percent in the first primaries surged in the runoffs as they become better known, while views of incumbents tend to be fixed and unchanging.

Last month, veteran Rep. Ralph Hall, R-Texas, lost the second time around to former U.S. Attorney John Ratcliffe. Hall had outpolled Ratcliffe by 45-29 percent in a March primary. Head to head, Ratcliffe won with 53 percent.

His victory continued a trend. Two years ago, Ted Cruz, then a Houston attorney, came in second to Lt. Gov. David Dewhurst by 9 percentage points in the first primary. Fueled by tea party momentum, Cruz picked up virtually all the anti-Dewhurst voters and easily beat the establishment pick in the runoff.

In 2004, Jim DeMint, then a congressman, at first got 26 percent in his Senate nomination bid, finishing 10 points behind former Gov. David Beasley. DeMint more than doubled that total in the runoff and won in a landslide.

This wasn’t the way runoffs were supposed to work.

The system evolved in the late 19th century, when Democrats were starting to dominate the post-Reconstruction South, and it worked for nearly a century. Virtually no Republicans were elected statewide in the former Confederate states for decades.

Direct election of senators became part of the Constitution in 1913, and Southern states tended to send senators to Washington who stayed to become committee chairmen or majority leaders. Those positions not only meant more dollars and federal jobs for their states, they also made it difficult to enact civil rights laws.

“We never liked Washington, but we said we will send power players up there and show them who’s boss,” said Marty Wiseman, former executive director of the Stennis Institute, a government studies center at Mississippi State University.

Runoffs were also important because they helped do away with fringe candidates or those the bosses couldn’t trust, said Merle Black, a co-author of “Politics and Society in the South.” The Republican Party was rarely a factor in the South until the 1960s, meaning officeholders were effectively chosen in Democratic primaries.

White supremacy was a convenient unifier. Though few blacks were able to vote in the South in those days, the Democratic bosses thought it was important to suppress that vote anyway. Blacks had majorities in South Carolina, Louisiana and Mississippi in the early 1900s, and even if only a few of them voted, they might otherwise make a difference.

By the 1960s, barriers to black voting were breaking down, and the bosses faced a new kind of trouble. The South was turning Republican, and runoffs began to acquire a new meaning. While most Southern Republicans are white, the split this time was between ideological and mainstream conservatives.

The elites thought the system was tilted in their favor. South Carolina would hold a runoff two weeks after the primary. Mississippi’s second Senate round is being held three weeks after the primary _ the thinking being that a challenger would lack the money, name recognition and organizational skill to regroup quickly.

That thinking now often proves wrong. The day after Mississippi’s June 3 primary, independent third-party groups began plotting strategy. They already had spent $5 million to boost McDaniel, and they were ready to spend plenty more.

There’s an irony to all this: Runoffs in the South tend to help unify the Republican Party, making it hard for Democrats to won. The overwhelming majority of Southern black voters are Democrats.

White supremacy is no longer the issue, said Kousser. Sen. Tim Scott, a Republican who’s black, appears headed to an easy re-election in South Carolina.

But, Kousser said, “to the extent that the Republican runoff is considered the election, it undercuts the Democratic Party.”

And that, he said, means “It also prevents the now-dominant majority of the Democratic Party in many states, African-Americans and/or Latinos, from gaining ultimate political power.”